Variable reluctance sensors are used to monitor both the speed of rotating shafts and the torque loading on shafts in gas turbine engines and gear boxes connected to gas turbine engines. For example, FIG. 1 illustrates an assembly for monitoring the torque loading on a power transmission shaft between a gas turbine and the power gearbox that drives the propeller.
When a load is applied to a power transmission shaft it will twist. For a known modulus of elasticity and at a constant temperature, the amount of twist (A) is proportional to the torque transmitted. This basic principle is used to measure torque.
The assembly shown in FIG. 1a comprises two intermeshed phonic wheels 10, 11, attached to the rotating shaft 12 but at points longitudinally spaced from each other. The torque transmitted by the shaft is calculated by measuring the time difference between the passage of the teeth of the two phonic wheels past a variable reluctance sensor. FIG. 1b illustrates a sensor 14 positioned adjacent to the teeth of a phonic wheel 17.
From FIG. 1a, it can be seen that the phase wheel 10 is attached directly to the shaft 12. The reference wheel 11 is attached to a reference tube 13 mounted concentric with the torque shaft 12, and is fixed to the torque shaft at one end leaving the reference wheel 11 free. When the shaft 12 is loaded it will twist but the unloaded reference tube 13 will not, so that the phase wheel 10 moves relative to the reference wheel 11. As a result, the reference wheel becomes a datum from which to calculate the angle of twist, θ. As the phonic wheels 10, 11 are intermeshed movement of the phonic wheels with respect to each other will be evident by the time intervals of the passage of phonic teeth on the phonic wheels past the sensor. The teeth on the phase phonic wheel will move closer to the teeth on the reference phonic wheel in the direction of rotation and twist. At the same time the distance between the trailing teeth of the reference phonic wheel in respect to the phase phonic wheel teeth will increase. This is illustrated in FIGS. 2a and 2b, which are schematic representations of the relative positions of the teeth on each wheel in an unloaded state and a loaded state respectively. The distance of ‘tm’ is always smaller than ‘ts’ so that the control system can differentiate between ‘tm’ and ‘ts’ when the phonic wheels start to rotate.
The distance between the phonic wheel teeth will be seen as the distance between the zero crossovers in the A/C signal produced by the variable reluctance sensor. The change in distance in the zero cross over will be directly proportional to the angle of the twist of the shaft (θ) and so the torque transmitted by the shaft. A typical clean signal waveform from a variable reluctance sensor sensing the passage of the teeth can be seen in FIG. 4, with time on the x-axis and voltage on the y-axis.
The same basic principle is equally applicable for the measurement of rotational speed via a phonic wheel. The time between the passings of adjacent teeth past a sensor can be measured to provide a signal from which rotational speed can be calculated.
Both the conventional type of variable reluctance sensor, where many turns of a conductive wire are wrapped around a magnetic pole piece, and the transformer type as described in U.S. Pat. No. 7,148,679, where a few turns of a primary turn of conductive wire are wrapped around magnetic pole piece, can be used. FIG. 3 is a schematic cross section of a typical construction of a variable reluctance sensor.
The sensor of FIG. 3 comprises a magnetic pole piece 30 around which an electrically conductive wire 31 is wound. A permanent magnet 32 is positioned adjacent a back face 30a of the pole piece 30. The front face of the pole piece 30b is, in use, located proximate to the phonic wheel or wheels being sensed, as shown in FIG. 1b. The pole piece 30, conductive wire 31 and permanent magnet are all held in a housing 33. An encapsulation material 34, typically a powder or an epoxy resin, is used to fill the space between the housing 33 and the pole piece 30, magnet 32 and conductive wire 31. The housing 33 is fixed to another part of the turbine engine (not shown) and ensures that the front face of the pole piece is correctly positioned relative to the phonic wheel or wheels. The housing also provides protection from the harsh environment found inside gas turbine engines.
As each tooth of the phonic wheels passes close to the front face of the pole piece there is a change in the magnetic flux experienced by the conductive wire 31, due to the change in the reluctance of the magnetic circuit consisting of the pole piece 30, the phonic wheel and the air gap between the two. The changing magnetic flux results in a variable current induced in the conductive wire 31, from which the timing of the passage of the teeth on the phonic wheels past the pole piece can be determined.
In both torque and speed measurement, it is important that the waveform produced by the variable reluctance sensor is very clean and there is no noise or additional modulations, known as microphony, on the signal waveform. FIG. 4 illustrates a clean waveform. In contrast, FIG. 5 shows a waveform that is not acceptable as there is significant noise 50 present. If the noise amplitude exceeds the trigger threshold of the engine controls, the torque or speed measuring system will not function properly as the noise will be interpreted as an additional zero crossing, and in extreme circumstances the controls may shut the engine down if the torque or speed measurement is a primary engine function.
One major cause of noise in the output from variable reluctance sensors, producing the additional modulations or microphony, is vibration from the surrounding environment. Vibration can be created from many areas of a gas turbine engine and surrounding ancillary equipment, such as the power gear box where large intermeshing teeth create vibration, out of balance shafts, bearings and compressor/turbine blades and discs.
The reason that vibrations cause noise in the output signal is the affect that they have on the pole piece. Vibration in the sensor environment can cause stress in the pole piece that alters its magnetic permeability. The change of the magnetic permeability of a material when subjected to a mechanical stress is known as the Villari effect. The stress energy created in the pole piece causes strain, which affects the permeability and so alters the reluctance of the device. As the pole piece has conductive wires wrapped around it and a magnet or coil attached at one end, the change in reluctance will cause a change in the magnetic flux around the pole piece, inducing an additional electrical current in the conductive wire wrapped around the pole piece. This additional induced current is the source of noise or microphony in the output signal. This effect is more noticeable at high vibration frequency levels because of the greater rate of change of permeability of the pole piece.
A problem with existing sensors, as illustrated in FIG. 3, is that any forces exerted on the pole piece 30 by the magnet 32 and/or the surrounding encapsulation medium or the housing front face result in strain energy in the pole piece 30. This strain energy changes the permeability of the pole piece, creating EMF in the conductive wire 31, which produces additional, unwanted modulations in the waveform, as shown in FIG. 5.
In a sensor as illustrated in FIG. 3, the inventors have found that there are two main mechanisms by which strain is generated in the pole piece. First, vibration from the surrounding environment causes the magnet to vibrate. The permanent magnet is relatively massive and vibrations of the magnet produce stress in the pole piece as the magnet pushed against it, resulting in microphony in the coil. Second, vibration from the surrounding environment causes vibration of the housing front face, which is transferred to the pole piece as strain energy, resulting in microphony in the coil.
The encapsulation material does, to some extent, reduce the transfer of vibration to the pole piece, and epoxy resin as an encapsulation material has proven to be the most effective material. However, at high frequency and high temperature there is still significant noise in the sensor output as a result of environmental vibrations. One factor is that, at the high temperatures found in gas turbine engines, the epoxy resin used as an encapsulating material is relatively soft.
It is an object of the present invention to substantially reduce the sensitivity of variable reluctance sensors, suitable for use in gas turbine engines, to noise resulting from environmental vibrations.